1. Field of the Invention
This invention relates to a recording apparatus, and in particular to a serial type recording apparatus which uses a step motor as a drive source to affect the movement of at least a recording head for recording scanning.
2. Related Background Art
In a serial type recording apparatus according to the prior art, a hybrid type or PM (permanent magnet) type step motor or a brushless motor is often used as a carriage driving motor for conveying a recording head for recording scanning.
For example, in the brushless motor, a Hall element, for instance, is usually used for the detection of the positions of the magnetic poles of a rotor for affecting the control of electrical energization and an optical type or magnetic type encoder is used for the detection of the speed of the rotor.
However, such a brushless motor suffers from the following problems:
(1) The alignment of stator magnetic poles an the Hall element is necessary.
(2) If the switching of power supply is affected by the Hall element, the positions of the Hall element and the stator are primarily determined and therefore, the method of electrically energizing the motor is fixed. For example, between a case where so-called 180.degree. energization control is affected and a case where 90.degree. energization control is affected, the position of the Hall element becomes electrically different by 45.degree. relative to the magnetic poles of the stator and therefore, to affect two types of energization controls by a motor, the number of Hall elements must be doubled and the Hall elements must be disposed at positions suitable for the respective energization controls.
For example, Japanese Patent Application Laid-Open No. 62-193548 and Japanese Patent Application Laid-Open No. 62-193549 propose a stepping motor in which energization control is affected by the use of the output of an encoder, but what is disclosed therein is only the structure itself of a motor in which an encoder is provided at a predetermined location, and no disclosure is made of the circuit, method, etc. for the drive control, of the motor.
So, in U.S. Pat. No. 4,963,808, there is proposed a control device for a step motor in which an encoder having portions to be detected an integer number of times as many as the number of the magnetic poles of a rotor is fixed to the shaft of the rotor and at a predetermined location on the stator side, the number of the portions to be detected of said encoder resulting from the rotation of the rotor is counted, whereby the electrical energization of the coil of the stator is switched over when the count value coincides with a predetermined value. Heretofore, the drive control of a step motor has been affected by simply open-loop-controlling the frequency of the same pulse as the driving pulse number of the motor. However, where a step motor is used as a carriage driving motor and is driven under open loop control, when the carriage is driven for movement, particularly in the case of the hybrid type, there is produced a keen, harsh noise attributable to the vibration of the rotor of the step motor. Also, when the carriage is started, stopped and reversed, that is, when the step motor is started, stopped and reversed, there is produced a great noise "bang" because the step motor is started or stopped while vibrating. These noises pose a problem in a printer like an ink jet printer such as a bubble jet printer which produces very little noise.
Also, it is conceivable to use the aforementioned brushless motor as a carriage driving motor, but the brushless motor takes a long rising time during starting and is not suitable as a carriage driving motor which repeats starting, stoppage and reverse starting substantially for each line, and the use of the brushless motor cannot accomplish high-speed recording.
So, in U.S. Pat. No. 4,928,050, there is proposed a recording apparatus which uses a step motor as a drive source to move a recording head for recording scanning and which is provided with detecting means for detecting the rotational angular position of the rotor of said step motor, and control means for closed-loop-controlling the driving of said step motor in conformity with the result of the detection by said detecting means. In the closed loop control of this step motor, an encoder is mounted on the rotary shaft of the step motor, and the output signals of the encoder are counted, and when a predetermined count value is reached, a motor energization signal is changed over to thereby affect the control of the rotation of the step motor. For example, assuming that an encoder capable of outputting a signal of 288 pulses per one full rotation is mounted on a PM type step motor which makes one full rotation by 48 steps, there is obtained an encoder signal of 6 pulses per step. Accordingly, it is possible to rotate the step motor by changing over the energization signal for the step motor each time 6 pulses of the encoder signal are counted.
The reason why the step motor is thus controlled by closed loop is that chiefly by the closed loop control, the harsh noise attributable to the vibration of the rotor of the step motor during the driving for movement of the carriage when the step motor is driven and the great vibration sound produced when the carriage is started or stopped and reversed are prevented from occurring.
However, to closed-loop-control the step motor in the above-described example of the prior art, the encoder must be mounted within the motor to obtain a signal resulting from the rotation of the rotor, and this has led to an increased volume of the motor. Now that in recording apparatuses such as latest printers whose compactness and thinness are progressing, compactness and thinness are also required of motors for driving recording heads, such an increased volume of the motor has posed a serious problem.
Also, multicolor has recently been adopted in recording apparatuses, and where multicolor is adopted, it is necessary that the control of the printing position be affected highly accurately and where an encoder is mounted within the motor, there has been a limit in the detection of the printing position.